Python与设计模式（一）——Abstract Factory

最近在重新学习Python，想找一些优秀的Python源代码阅读学习，加上前段时间一直在学习设计模式，所以选择了GitHub上这个项目作为学习对象：

https://github.com/faif/python-patterns

这里有用Python实现各个设计模式的源代码demo。

本篇从Abstract Factory开始介绍。

其源码如下：

#!/usr/bin/env python
# -*- coding: utf-8 -*-

# http://ginstrom.com/scribbles/2007/10/08/design-patterns-python-style/

"""Implementation of the abstract factory pattern"""
import six
import abc
import random


class PetShop(object):

    """A pet shop"""

    def __init__(self, animal_factory=None):
        """pet_factory is our abstract factory.  We can set it at will."""

        self.pet_factory = animal_factory

    def show_pet(self):
        """Creates and shows a pet using the abstract factory"""

        pet = self.pet_factory.get_pet()
        print("We have a lovely {}".format(pet))
        print("It says {}".format(pet.speak()))
        print("We also have {}".format(self.pet_factory.get_food()))


# Stuff that our factory makes

class Dog(object):

    def speak(self):
        return "woof"

    def __str__(self):
        return "Dog"


class Cat(object):

    def speak(self):
        return "meow"

    def __str__(self):
        return "Cat"


# Factory classes

class DogFactory(object):

    def get_pet(self):
        return Dog()

    def get_food(self):
        return "dog food"


class CatFactory(object):

    def get_pet(self):
        return Cat()

    def get_food(self):
        return "cat food"


# Create the proper family
def get_factory():
    """Let's be dynamic!"""
    return random.choice([DogFactory, CatFactory])()


# Implementation 2 of an abstract factory
@six.add_metaclass(abc.ABCMeta)
class Pet(object):

    @classmethod
    def from_name(cls, name):
        for sub_cls in cls.__subclasses__():
            if name == sub_cls.__name__.lower():
                return sub_cls()

    @abc.abstractmethod
    def speak(self):
        """"""


class Kitty(Pet):
    def speak(self):
        return "Miao"


class Duck(Pet):
    def speak(self):
        return "Quak"


# Show pets with various factories
if __name__ == "__main__":
    for i in range(3):
        shop = PetShop(get_factory())
        shop.show_pet()
        print("=" * 20)

    for name0 in ["kitty", "duck"]:
        pet = Pet.from_name(name0)
        print("{}: {}".format(name0, pet.speak()))

### OUTPUT ###
# We have a lovely Cat
# It says meow
# We also have cat food
# ====================
# We have a lovely Dog
# It says woof
# We also have dog food
# ====================
# We have a lovely Cat
# It says meow
# We also have cat food
# ====================
# kitty: Miao
# duck: Quak

代码对应的类图如下：

大家可能会感到奇怪：这里并没有抽象工厂啊？我们可以看这段代码：

# Create the proper family
def get_factory():
    """Let's be dynamic!"""
    return random.choice([DogFactory, CatFactory])()

这里，其实就实现了一个抽象工厂的功能，生成目标的factory。DogFactory用来生产Dog对象，CatFactory用来生产Cat对象。Dog类和Cat类都是Pet类的子类。